1. Field of the Invention
The present invention relates to a microwave plasma generating apparatus and a process for the preparation of a diamond thin film utilizing the microwave plasma generating apparatus and, more particularly, to such a microwave plasma generating apparatus as capable of generating a large volume of microwave plasma in a stable fashion and to such a process for the preparation of a diamond thin film having a totally large area and a uniform quality by using the microwave plasma generating apparatus.
2. Description of Related Art
Microwave plasma generating apparatuses are of the type which are designed so as to generate plasma from a source gas by radiating microwaves in an electric discharge space filled with the source gas.
Conventional microwave plasma generating apparatuses of this type suffer from the disadvantage that plasma cannot be rendered of a large volume due to a limited electric power when a single electric power plant is employed.
In order to improve this disadvantage, a technique of introducing microwaves from multiple directions into a space in which electric discharge takes place has been developed, as disclosed in Japanese Patent Unexamined Publication (kokai) No. 24,094/1989, however, this technique has the drawback that plasma of a large volume cannot be generated in a stable fashion for a long period of time because the plasma generated may migrate due to interference of microwaves with each other and changes in a phase difference.
On account of this drawback, the use of such a microwave plasma generating apparatus for the process for preparing a diamond thin film cannot result in the formation of a diamond thin film having a uniform film thickness on a substrate of a large area.
FIG. 12 illustrates a conventional microwave plasma generating apparatus, comprising a quartz tube 13 within which a silicone wafer is set in a predetermined position, two pairs of a rectangular-sectioned waveguide 1 and a rectangular-sectioned waveguide 16 with a built-in reflector (short plunger) are concentrically disposed around and outside the outer periphery of the quartz tube 13 so as for each of the rectangular-sectioned waveguides 1 and 1 to face each of the corresponding rectangular-sectioned waveguides 16 and 16 with the built-in reflectors. In this illustration, each of the rectangular-sectioned waveguides is disposed so as for its rectangular opening in section to allow its two longer opposite sides to lie in a transverse direction, i.e., in a direction horizontal to the normal axis of the quartz tube 13, while allowing its two shorter opposite sides to lie in a longitudinal direction, i.e., in a direction parallel to the axis of the quartz tube 13. In other words, the direction of the electric field Ex1 and Ex2 of the microwave radiated between the pair of the rectangular-sectioned waveguides 1 and 16 is the same as the direction of the electric field Ey1 and Ey2 of the microwave radiated between the other pair of the rectangular-sectioned waveguides 1 and 16. The disposition of the electric fields in the same directions cannot stably generate a large volume of plasma for a long period of time.